It is known to measure voltage and current to ascertain the power consumption from a supply typically an alternating current supply, by generating signals corresponding to the voltage and current, producing a product of the two signals and integrating the result over a period of time.
In recent years development work has been concentrated on meters incorporating solid state devices for performing some of the computations. Potentially such devices allow more accurate measurement than older electromechanical designs of meter and will also enable remote reading to be incorporated more readily into such meters.
Initially attempts were made to multiply analogue signals relating to current and voltage. Circuits adapted to accurately handle analogue signals and to multiply same, are complex and expensive and prone to drift with temperature and age which mitigate against accuracy.
European Patent Specification No. 0181719 discloses an alternative design in which the voltage and current signal are sampled and digitised and supplied as digital input signals to a microprocessor for multiplication within the processor to produce a digital signal whose value is proportional to the product of the input signals. In order to achieve higher accuracy than would otherwise be possible using eight bit A/D conversion on (or off) the processor, an alternating ramp voltage is generated and added to the input signals.
Although digital multiplication replaces the analogue multiplication of the earlier designs, the device described in EP 0181719 suffers from the disadvantage that an alternating ramp voltage has to be accurately generated and the combination requires the handling of analogue signals with all the inherent instability of analogue circuit brought about by variation due to temperature, humidity, age etc. It is an object of the present invention to provide a power measuring apparatus which does not require the generation and compensation of analogue signals before the signals are digitised but which nevertheless possesses at least the same level of accuracy as the apparatus described in European Patent Specification No. 0181719.
It is a further object of the invention to provide power measuring apparatus which can readily be isolated electrically from the supply to enable user access ports (such as a coin freed or card reader control) to be associated therewith.
It is a further object of the invention to provide power measuring apparatus from which information can be derived using telemetry or other remote sensing techniques to permit remote meter reading.
It is a further object of the invention to provide a method of measuring power which does not require the generation and accommodation of analogue signals before the digitisation of current and voltage signals for multiplication.